1. Field of the Invention
The present invention relates to an image forming apparatus capable of, according to an electrophotographic method, forming an electrostatic latent image on a photosensitive drum with a laser beam scanning on the drum surface and developing a visible image on a recording medium by applying toner to the electrostatic latent image.
2. Description of the Related Art
An image forming apparatus configured to form an electrostatic latent image on a photosensitive drum and develop a visible toner image on a recording medium according to an electrophotographic method generally performs scanning with a laser beam on the photosensitive drum to form an electrostatic latent image on the drum surface.
An example method for forming an electrostatic latent image is described below with reference to FIG. 2. A photosensitive member 107 (photosensitive drum) can rotate around a horizontal axis thereof in a direction indicated by an arrow 308 at timing synchronized with completion of one cycle of a laser scanning operation. A polygonal mirror 102 can rotate around a vertical axis thereof in a direction indicated by an arrow 102A at a constant angular speed. A laser beam 301 emitted from a semiconductor laser generator 101 is incident on a reflection surface of the polygonal mirror 102. When the polygonal mirror 102 is rotating around its vertical axis, scanning of the laser beam 301 on the photosensitive member 107 can be performed along a scanning line indicated by an arrow 303. A beam detection (BD) sensor 302, placed on an upstream side of the photosensitive member 107, detects the laser beam 301 reflected (deflected) by the polygonal mirror 102. The electrostatic latent image writing method includes starting a laser driving operation based on image data when the laser beam 301 reaches an image writing start position 309 and forming an electrostatic latent image on the photosensitive member 107 within a predetermined region 307.
The electrostatic latent image writing method includes rotating the photosensitive member 107 in the direction of the arrow 308 by a predetermined amount (e.g., 42.33 μs) when a rotational angle of the polygonal mirror 102 exceeds a predetermined angle and restarting the scanning of the laser beam 301 for the next line on the photosensitive member 107 with another (next) reflection surface of the polygonal mirror 102. In this case, to adjust the image writing start position 309 for image data of each line, accurately measuring a time required for the laser beam 301 to travel from the BD sensor 302 to the image writing start position 309 is required, as discussed in Japanese Patent Application Laid-Open No. 2006-251513 and Japanese Patent Application Laid-Open No. 7-72400.
However, image data that is used to drive the semiconductor laser generator 101 and a rotation of the rotary polygonal mirror 102 that performs scanning of the laser beam 301 are in an asynchronous relationship. In this respect, there is a conventional method for obtaining a beam detection signal from the BD sensor 302 according to a high-speed clock and generating a synchronization (sync) clock, which is N demultiplied in frequency referring to the detection timing, as an image clock. However, according to this conventional method, various circuitry requirements need to be satisfied to modify the clock. The circuit using a high-speed clock is complicated in both designing and operational aspects.